1. Field of the Invention
The present invention relates to a device for carrying a printing head used in a printer or the like.
2. Description of the prior Art
FIG. 5 shows an example of a conventional device for carrying a printing head in the prior art. Referring to FIG. 5, in the printing head carrying device, there are provided a first guide shaft 2 disposed in parallel with a platen 1, a second guide shaft 3 disposed in parallel with the guide shaft 2, and a carriage 5 mounting a printing head 4 thereon and being slidably supported by the first and second guide shafts 2 and 3 such that the printing head is opposed to the platen 1. The carriage 5 is provided with a through hole 6 through which the guide shaft 2 is passed, and a set of a projecting portion 5a and a pressing plate 7 which sandwiches the guide shaft 3 therebetween. The carriage 5 is further provided with a support groove 5b and a protrusion 5c, and the pressing plate 7 is provided with protrusions 7a and 7b. The support groove 5b and the protrusion 7a are engaged with each other and a spring 8 is hooked between the protrusions 5c and 7b, so that holding force is applied onto the projecting portion 5a and the pressing plate 7.
In the foregoing prior art structure, a dimensional error of the guide shafts and the carriage has produced a clearance between the first guide shaft and the through hole through which the first guide shaft is passed and which is formed in the carriage. Particularly, recently, because plastic or the like is used as a material for the carriage and injection molding or the like is employed as the method of forming the carriage, a dimensional error in forming the carriage is large and wear between the carriage and the guide shafts sliding movement becomes a serious factor for making the clearance large, resulting in a large obstacle to the improvement in printing quality.
Namely, in the state where printing is not performed, the printing head is normally lowered by a quantity of the clearance by the gravitational force acting onto the printing head and the carriage. If printing is started, however, the printing force of the printing head against the platen causes the carriage to float and hence the printing head is also caused to float as the carriage floats. The quantity of floating is not fixed but changed owing to various factors such as the number and the positions of dots to be formed, a carrying speed of the printing head, and so on. Further, if the carriage is caused to slide while changing the quantity of floating, the sliding resistance changes irregularly, so that the driving speed of the printing head at the different positions becomes fluctuated. Accordingly, in the characters or the like printed on a recording paper, a waving phenomena is caused to thereby lower the printing quality.
In order to solve such the problems, the clearance should be prevented from being produced. To this end, however, it is necessary to finish the carriage and the guide shafts extremely accurately, so that the carriage and the printing head have been limited in structure, and the material and the weight of them have been also limited because of countermeasure against wear. Accordingly, the cost has become extremely high and the design has been much limited.